This invention relates to circuits for establishing a reference level in a video signal which may include unwanted low frequency components.
In producing a video display, it is important that certain portions of the video signal be at standard reference voltage levels. The video signal as received may depart from these levels either through various sources of error or because of a slowly varying backbround signal intentionally introduced upon transmission of the signal. It is known in the art that proper reference levels may be established by first passing the signal through A.C. coupling, then clamping the signal to a selected voltage value precisely at times when the signal should assume that value.
Conventional practice is to clamp the video waveform during a portion of the standard horizontal line synchronizing pulse known as the sync tip, the tip of the synchronizing pulse. If the waveform has been subject to distortion so that the sync tip has the wrong pulse height relative to the other portions of the waveform, then the whole waveform will be displaced when the sync tip is used as a reference level. Therefore, when the sync tip is used for clamping, circuitry is required which will first assure the proper height of the sync tip.